This invention relates to the construction and design of a plastic optical device. More particularly, it relates to a plastic contrast enhancement filter useful for the reduction of specular glare from the surface of cathode ray tubes, computer display screens and the like.
The construction of a contrast enhancement filter, such as can be used in front of the faceplate of a cathode ray tube or computer display screen for enhancement of contrast and reduction of specular glare, has been known. Such a device is described, for example, in the U.S. Pat. No. 4,278,736, issued July 14, 1981, to M. A. Kamerling. Typically, a contrast enhancement filter suited to such applications will comprise a plastic sheet-like circular polarizing element as part of a multilayer laminated article. In the aforementioned U.S. Pat. No. 4,278,736, there is described a contrast enhancement filter which includes a pair of glass support members and which has a plastic light-polarizer laminated therebetween into a unitary assembly. Contrast enhancement filters comprising a lamination of a plastic circular polarizer element and a plastic sheet member carrying an antireflection layer have also been known and examples thereof have been commerically available from Polaroid Corporation under the trade designation CP-50 and CP-Custom.
In the manufacture of an optical sheet element, it will often times be advantageous to provide the outermost surface of the element, which faces the viewer, with an antireflection layer to minimize specular glare. The application of optical thin films to substrate materials for various purposes has been well known and the properties of, for example, dielectric thin films are reported by H. K. Pulker in Applied Optics. Vol. 18, No. 12, June 15, 1979. Frequently, the optical and structural properties desired in a particular thin film application will be influenced by the substrate temperature and the conditions required to suitably deposit the desired film. For example, the microstructure and integrity of an optical thin film coating is oftentimes enhanced by application of the film onto a heated substrate or by annealing the coated article. While a glass substrate material will normally be suited to manufacturing methods which involve application of coatings at elevated temperatures and may withstand an annealing treatment, various applications will be promoted by the use of plastic substrate materials, particularly, thin flexible plastic sheet materials, which are not adapted to such methods or treatments. Consequently, the nature of the substrate material will unfortunately restrict the choice of available materials which otherwise would be suited to a particular optical application.
In the production and use of plastic sheet elements of various types, the tendency of plastic sheet materials to exhibit static build-up and discharge has been recognized. In the case of a contrast enhancement filter, in position over the faceplate of, for example, a computer display screen, the build-up of static electricity, and the associated attraction of dust particles and required cleaning, may be annoying or distracting. It will, thus, be appreciated that it will be advantageous if there is provided a contrast enhancement filter which may be readily manufactured from low-cost plastic components and which exhibits good antistatic and antireflection properties.